1. Field of the Invention
The invention relates in general to a thin film transistor (TFT) structure and method of manufacturing the same, and more particularly to the thin film transistor structure capable of reducing current leakage and method of manufacturing the same.
2. Description of the Related Art
Conventional computer systems have used cathode ray tube (CRT) technology for the computer display. However, in the past decade technological advances in thin film transistor (TFT) liquid crystal displays (LCDs) have made this an increasingly popular technology. The active matrix thin film transistor liquid crystal display (AM TFT-LCD) possesses several competitive advantages, including lighter weight and thinner profile. The consumers also demand the perfect images displayed on the AM TFT-LCD. The higher resolution of the display is achieved, the larger number of the scan lines is required. Also, each gate scan line has to be selected in a shorter time under the same frame time. The requirement of the on-and-off characteristic of the TFT is stricter for improving the performance of the display. For example, excess leakage current is not allowed when the TFT is in the off state.
In a TFT manufacturing process, the amorphous silicon (a-Si) layer is photosensitive and is likely to generate photocurrent and cause photocurrent leakage after being exposed to the light. For preventing the photocurrent leakage, an island-in structure of the TFT, which the pattern of amorphous silicon is restricted within the pattern of the gate electrode (i.e. a-Si fully blocked by the gate electrode), is provided.
FIG. 1 is a cross-sectional view partially showing a conventional thin film transistor (TFT) device having an island-in structure. The TFT 1 includes a substrate 10, a gate electrode 11 formed on the substrate 10 (by patterning the first metal layer), a gate insulation layer (such as a silicon nitride layer) 12, an amorphous silicon (a-Si) layer 13, a doped amorphous silicon (such as an n+ a-Si layer) 14, the source and drain regions 151, 152 (both formed by patterning the second metal layer). Also, the source region 151 and the drain region 152 are separated by a channel 16. The doped amorphous silicon layer 14 is formed for the purpose of increasing ohmic contact between the source region 151/the drain region 152 and the a-Si layer 13. Typically, the a-Si layer 13 and the doped a-Si layer 14 are so-called as the island structure. In FIG. 1, the bottom area of the a-Si layer 13 is smaller than that of the gate electrode 11, so that the TFT 1 presents an island-in structure.
According to the island-in structure of the TFT of FIG. 1, the a-Si layer 13 is shielded by the gate electrode 11, and no photocurrent leakage occurs when the TFT 1 is illuminated. However, the source region 151 and the drain region 152 (both made of the metal) direct contact the island structure (including the a-Si layer 13 and the doped a-Si layer 14), and this direct contact of a metal with a semiconductor is known as “Schottky contact”. A leakage current path is thus formed. When a potential difference exists between the gate electrode 11 and the drain region 152, the current leakage is observed due to the “Schottky contact”, and has the effect on the on-and-off characteristic of the TFT 1.